1.0 Field Of The Invention
The present invention is generally directed to a process for repulping wet strength paper in which the reaction is conducted in a two step process wherein the initial breakdown of the wet strength paper to fibers is conducted at a first pH value and the remainder of the process is conducted at a second, higher pH value until the conversion to fibers is substantially complete.
2.0 Description Of Related Art
Wet strength paper contains a resin which is adsorbed onto paper fibers during the papermaking process and cross-links on heating or aging of the paper to form a polymeric network which adds strength to the paper. Wet strength resins fall into two groups, permanent and temporary, and include formaldehyde-based resins, amine-epichlorohydrin based resins and aldehyde polymers.
In order to repulp wet strength paper, it is necessary to hydrolyze the cured resin. For many years metal hypochlorites (e.g. sodium and potassium hypochlorite) have been used as reagents for repulping wet strength paper as shown by, for example, C. S. Maxwell, U.S. Pat. No. 3,407,113 and H. R. Miller, U.S. Pat. No. 3,427,217.
Hypochlorites generate adsorbable organic halides in paper mill effluents. These substances are environmentally undesirable. Accordingly, the papermaking industry has sought to use non-chlorinated reagents for repulping wet strength paper.
Alkali metal persulfates (M.sub.2 S.sub.2 O.sub.8) and peroxymonosulfates (MHSO.sub.5) have recently been used as reagents to replace hypochlorites. H. H. Espy et al., "Persulfates as Repulping Reagents for Neutral/Alkaline Wet-strength Broke", Tappi Journal, Vol. 76, No. 2, pp. 139-141 (February 1993) and P. C. Kapadia et al., "A Non-Chlorine Repulping Aide", (1992 Tappi Papermakers Conference) show significant reductions in organic chlorides using peracid salts.
Peroxide compounds alone, such as hydrogen peroxide and peracetic acid, and combined with non-alkali, non-alkaline earth metal salts and/or metal chelates have also been proposed as repulping reagents as disclosed in J. Sajbel et al., U.S. patent application Ser. No. 08/112,779 filed on Aug. 26, 1993, the entire disclosure of which is incorporated herein by reference.
Repulping reagents, especially persulfates and peroxymonosulfates, are typically employed in a single pH environment, most commonly under strongly alkaline conditions (i.e. pH of 10-13) at temperatures of at least 50.degree. C. However, these process conditions do not provide a high level of repulping, especially when repulping bleached paper with high levels of wet strength resin and unbleached paper broke or corrugated containers.
There is therefore a continuing need to improve the process of repulping paper using environmentally acceptable reagents which are effective in repulping wet strength paper, especially for the repulping of bleached paper and unbleached paper broke or corrugated containers.